Method of soldering a polymer surface to a conducting or semiconducting surface and applications of same

ABSTRACT

The invention relates to a method of bonding a polymer surface to an electrically conductive or semiconductive surface, which method is characterized in that it comprises: a) the electrografting of an organic film onto the conductive or semiconductive surface; and then b) an operation of bonding the polymer surface to the conductive or semiconductive surface thus grafted. It also relates to applications of this method and to structures obtained by its implementation.

TECHNICAL FIELD

The present invention relates to a method for bonding a polymer surfaceto a conductive or semiconductive surface, to applications of thismethod and to structures obtained by implementing it.

The term “polymer surface” is understood to mean a surface formed from apolymer and corresponding to all or part of the surface of an objectthat may be either exclusively made from this polymer or formed from oneor more other materials, at least one part of the surface of whichconsists of said polymer. In particular, the object may be an objectthat includes an electrically conductive or semiconductive region, thesurface of which consists of a polymer.

The expression “conductive or semiconductive surface” is understood tomean a surface consisting of an electrically conductive orsemiconductive material and corresponding completely or partly to thesurface of an object that may be either exclusively formed from thisconductive or semiconductive material or is formed from one or moreother materials, at least one part of the surface of which consists ofsaid conductive or semiconductive material.

The method according to the invention is of course applicable in allfields in which it is necessary to bond a polymer, and in particular athermoplastic polymer, to an electrically conductive or semiconductivesurface.

For example, mention may be made of the field of composites, such asthose used in the aerospace, aeronautical and automotive industries inwhich the problems of corrosion-protection paints and coatings on partsflaking off for example can be remedied by the method of the invention.Mention may also be made of the biomedical field in which the methodaccording to the invention may be useful, for example, for coatingmedical or implantable surgical instruments such as vascularendoprostheses (or stents), aneurysm guides, catheter guides,pacemakers, hip prostheses, cochlear implant electrodes, dental implantsor even electrophysiology electrodes, with biocompatible materialssuitable for ensuring controlled release of biologically activesubstances.

However, the method according to the invention may also be applicable infields where it is necessary for there to be strong mechanicalattachment between two objects having electrically conductive orsemiconductive regions, and especially when the attachment has to beeffected in these regions. In particular, the method according to theinvention may be of great use in the case in which it is desirable forthe attachment to be carried out at temperatures below those needed foreffecting direct thermal bonding, whether for technical reasons (such asthe heat sensitivity of the materials) or economic reasons, or else whenthe attachment is intended to be carried out via a flexible link. Suchconstraints are very commonly present in the assembly of sensitivecomponents of microsystems, such as microsensors, and in particular inthe assembly carried out using the PFC (polymer flip chip) technology.

PRIOR ART

The bonding of an organic material, and especially a polymer, to anelectrically conductive or semiconductive material poses a number ofdifficulties.

This is because organic materials exhibit localized surface states: inchemical terms, they are said to possess functional groups. In thiscontext, it is “simple” to carry out chemistry on a polymer surfaceinsofar as this involves making the functional groups react together andtherefore draws on experience acquired in organic chemistry. However,the surface of conductive or semiconductive materials is formed fromdelocalized electronic states (excluding surface defects): in chemicalterms, even the notion of functional groups disappears and experienceacquired in organic chemistry can no longer be applied to the surfacechemistry of conductive or semiconductive materials.

Thus, included among the proposed solutions are firstly those consistingin providing the surface of the conductive or semiconductive materialwith organic functional groups so that this material can be bonded tothe organic material by a chemical reaction.

This may for example be achieved by forming, on this surface, layers ofoxides or hydroxides on which it is then possible to make complementaryfunctional groups react, such as isocyanate groups (EP-A-1 110 946 [1]),siloxanes (WO-A-00/51732 [2]) or acid chlorides (FR-A-2 781 232 [3]), orusing bifunctional coupling agents or chemical adhesion promoters suchas γ-aminopropyltrimethoxysilane (E. P. Plueddmann in “Fundamentals ofAdhesion”, L. H. Lee (Editor), page 269, Plenum Press, New York, 1990[4]).

The surface of the conductive or semiconductive material may also bepretreated so as to create functional groups thereon that have a higherreactivity than that of the abovementioned oxides and hydroxides, with aview to obtaining a more rapid reaction with the polymer material. Thesemay especially be unstable functional groups, formed in a transientmanner, such as radicals generated by an abrupt oxidation of saidsurface, either by chemical means or by irradiation.

Thus, for example, U.S. Pat. No. 6,022,597 [5] proposes the exposure ofthe surface to a reactant having nitrogen-generating groups (for exampleazide groups) and bombardment of this surface by particles (ions,electrons, protons, etc.) in order to convert the nitrogen-generatinggroups into nitrene groups capable of subsequently reacting with a largenumber of organic functional groups.

U.S. Pat. No. 6,287,687 [6] proposes the functionalization of a surfaceby subjecting it to a plasma treatment in which the plasma gas containsa monomer capable of polymerizing or copolymerizing with other compoundsthat can polymerize under irradiation.

U.S. Pat. No. 4,421,569 [7] proposes to functionalize a surface byapplying an aqueous suspension, comprising a polymer precursor monomer,a prepolymer, metal salts and a catalyst, to said surface, the metalsalts being used to oxidize said surface so as to create thereonradicals that can initiate the monomer and prepolymer polymerizationreactions.

The surface of the conductive or semiconductive material may also befunctionalized by means of radicals that are made to react byirradiation with heavy ions (U.S. Pat. No. 6,306,975 [8]), by thermalmeans (WO-A-98/49206 [9]) or even photochemically as disclosed inWO-A-99/16907 [10].

All these methods rely on the intention of creating the strongestpossible chemical bonds, and especially covalent bonds, at the organicmaterial/conductive or semiconductive material interface. In general,these methods have the two disadvantages of using chemical reactantsand/or activation operations that are complex and expensive, and ofrequiring the optimization of operating protocols that are oftenlengthy, since the surface chemical reactions occur at substantiallylower rates than equivalent rates in solution.

Moreover, methods using polymerization reactions initiated from thesurface of a material make it possible, at the end of reaction, toremove neither the initiators nor certain monomers that have notreacted. Now, many of these compounds are toxic, because they arereactive, which makes this type of method unsuitable for use in thebiomedical field.

In addition, insofar as the material that has to be attached to thefunctional groups provided on the surface of the conductive orsemiconductive material itself acts as a chemical reactant, it is oftennecessary to chemically modify said material in order to attach to itgroups that are complementary to those present on said surface. Thisagain adds, to the operating protocols, at least one additional step andsubstantially increases its cost.

Another type of solution consists not in creating chemical bonds at theorganic material/conductive or semiconductive material interface but informing, on the conductive or semiconductive surface, a layer of theorganic material, which is insoluble in most known solvents, byreckoning on the fact that physical forces present are sufficient toensure adhesion of this organic material to said surface, as long as theinterface is stable.

This can be achieved in particular by crosslinking the organic materialduring or after its deposition on the surface of the conductive orsemiconductive material, it being possible for this deposition to becarried out by spin coating or by dip coating.

This approach is used in the biomedical field, for example for coatingstents with polymeric reservoirs of active molecules, which aresubsequently stabilized by crosslinking of fibrin (EP-A-0 701 802 [11])or of a chemical crosslinking agent (WO-A-98/32474 [12]), and in thefield of Microsystems for producing polymer bumps used for themechanical assembly of microstructures via the PFC technology, or elseused for the encapsulation of Microsystems (U.S. Pat. No. 6,335,571[13]).

However, this approach has the major disadvantage of resulting, owing tothe absence of bonds at the interface between the organic material andthe conductive or semiconductive material, in joints that aremechanically not very strong, especially when they are subjected tostresses of the vibration, torsion or similar type, in particular at theorganic material/conductive or semiconductive material interface.

Even though the PFC technology has developed considerably, manyassemblies are still produced using the “indium bump or flip-chipconnection” technique that uses fusible metal bumps based on lead andindium (FR-A-2 780 200 [14]). In general, this type of assembly requireshigh bonding temperatures and also uses mechanically weak joints becausethey can break when stressed.

Finally, it should be emphasized that, in the case of the assembly ofmicrostructures, none of the abovementioned methods is suitable for usein restricted conductive or semiconductive regions. This is because,whether attachment is via chemical surface reactions, crosslinkedpolymers or fusible bumps, it is necessary in all cases, during at leastone step, to carry out a conformal deposition process, in which thematerial deposited conforms to the topology of the conductive orsemiconductive regions, such as the use of dispensing robots capable ofpipetting a polymer solution at the desired point or else laser ablationoperations capable of removing a layer of polymer that has beenuniformly deposited on the surface.

It follows from the foregoing that there exists a real need for a methodthat allows a polymer material to be firmly attached to a conductive orsemiconductive surface while being free of the drawbacks of the variousmethods proposed hitherto for carrying out such an attachment.

SUMMARY OF THE INVENTION

The present invention makes it possible to meet this requirement byproviding a method of bonding a polymer surface to a conductive orsemiconductive surface, which method is characterized in that itcomprises:

-   -   a) the electrografting of an organic film onto the conductive or        semiconductive surface, and then    -   b) an operation of bonding the polymer surface to the conductive        or semiconductive surface thus grafted.

Within the context of the present invention, the expression“electrografting of an organic film onto a conductive or semiconductivesurface” is understood to mean an operation that consists in bringingthis surface into contact with at least one precursor of this organicfilm and in causing, by applying one or more electrical potential scansto the conductive or semiconductive surface, this precursor to beattached via covalent bonds to said surface and, thereby, forming anorganic film.

The attachment of the precursor to the conductive or semiconductivesurface may, when said precursor lends itself thereto, be accompanied byprecursor polymerization reactions that have the effect in particular ofincreasing the thickness of said organic film.

Moreover, the expression “operation of bonding a polymer surface to aconductive or semiconductive surface” is understood to mean an operationconsisting in bonding these two surfaces so that they form anindivisible mass.

As will be described later, this operation may be carried out equallywell cold, for example by means of a substance capable of dissolving orswelling the polymer surface and the organic film electrografted ontothe conductive or semiconductive surface—this type of operation beingreferred to hereafter by the term “cold bonding”—as hot, i.e. bysupplying thermal energy suitable for causing the contacting surfaces tomelt—this type of operation being called hereafter “hotmelt bonding”, oreven by combining cold bonding with hotmelt bonding.

Before carrying out this bonding operation, the method according to theinvention includes subjecting the conductive or semiconductive surfaceto a pretreatment, which consists of the electrografting of an organicfilm, the film thus electrografted having the two advantages of beinghighly adherent to the surface that has given rise to it and of beingorganic, like the polymer surface that has to be bonded to theconductive or semiconductive surface.

Thus, although the thickness of this film is generally small to verysmall (i.e. less than 1 μm or even less than 500 nm), it turns out,surprisingly, that said film is capable of acting as a bonding “seed”and of making it possible to form a bonded joint between the conductiveor semiconductive surface and a polymer surface by simple contactbetween the latter and by applying bonding conditions.

Thus, although it is impossible ordinarily to bond a polymer materialdirectly to a conductive or semiconductive material, owing to theirdifference in nature and, thereby, difference in melting points,difference in bonding stresses, etc., such bonding is made possible bythe method according to the invention thanks to a prior modification ofthe surface of the conductive or semiconductive material, thismodification consisting in the electrografting of an organic film.

According to the invention, the organic film may be electrografted ontothe conductive or semiconductive surface by electroinitiated grafting orelectrocontrolled deposition, in which case the organic film is apolymer film.

In the case of electroinitiated grafting, it is only the attachment ofthe precursor to the conductive or semiconductive surface that resultsfrom an electrochemical reaction, that is to say a reaction caused bythe application of an electrical potential, the precursor polymerizationreactions, when they exist, being purely chemical, autonomous andindependent of any electrical potential.

However, in the case of electrocontrolled deposition, the precursorpolymerization reactions are electrochemical, like the attachment ofthis precursor to the conductive or semiconductive surface, andtherefore remain bonded by maintaining an electrical potential. Anexample of electrocontrolled deposition is electropolymerization thatuses, as precursors, conductive monomers such as pyrrole, aniline,thiophene or EDOT (ethylene dioxythiophene).

Within the context of the present invention, it is preferred that theelectrografting of the polymer organic film be electroinitiatedgrafting. This is because the inventors have found that this type ofelectrografting has the following advantages: (i) it results in theformation of covalent bonds between the polymer organic film and theconductive or semiconductive surface; (ii) it allows polymer to bedeposited extremely locally on chosen areas having a given workfunction; (iii) it permits very precise control over the thicknessuniformity, even on highly uneven ohmic-drop topographies (roughsurfaces, worked surfaces having high aspect ratio features, etc.).

According to a first preferred way of implementing the method accordingto the invention, when the organic film is a polymer film, the precursoris a monomer or a precursor prepolymer of this film, or else a mixtureof the two.

In this preferred method of implementation, the electrografting of thepolymer film comprises, apart from the attachment of the precursor ofthis film to the conductive or semiconductive surface, polymerizationreactions within the chain of this precursor. These polymerizationreactions take place from the monomers and/or prepolymers that areattached to said surface under the effect of the electrical potential,and results in growth or “sprouting” of polymeric chains from thissurface. Each polymeric chain thus formed is therefore covalently bondedto the conductive or semiconductive surface.

When the grafting is electroinitiated, the precursor monomers andprepolymers of the organic film may be chosen, in the first place, fromorganic compounds having vinyl groups, in which case theelectroinitiation consists of an electroreduction (or anelectrooxidation) of these monomers and/or prepolymers. It is these thuselectroreduced (or electrooxidized) monomers and/or prepolymers thatinitiate the polymerization reactions which, in this case, are anionic(or cationic).

The monomer compound or prepolymer that can be used for this purpose ispartly or completely functionalized by vinyl groups, and especiallyvinyl monomers such as acrylonitrile, methacrylonitrile, acrylates andmethacrylates (methyl acrylate and methyl methacrylate, ethyl acrylateand ethyl methacrylate, propyl acrylate and propyl methacrylate, butylacrylate and butyl methacrylate, hydroxyethyl acrylate and hydroxyethylmethacrylates, glycidyl acrylate and glycidyl methacrylate, polyethyleneglycol dimethacrylate, polydimethylsiloxane acrylate andpolydimethylsiloxane methacrylate), acrylamides and methacrylamides,cyano-acrylates, acrylic acid and methacrylic acid, styrene, vinylhalides, N-vinylpyrrolidone, 2-vinylpyridine, 4-vinylpyridine andvinyl-terminated telechelic compounds.

When the grafting is electroinitiated, the monomers and prepolymers mayalso be chosen from organic compounds containing cyclic groups that canbe cleaved by nucleophilic or electrophilic attack, in which case theelectrografting takes place according to the same principle as thatabove apart from the fact that the growth of the polymeric chainsresults from opening the monomer or prepolymer rings.

In this case, any monomer or prepolymer compound partly or completelyfunctionalized by cyclic groups cleavable by nucleophilic orelectrophilic attack can be used, especially epoxides, c-caproplactone,butyrolactone and telechelic compounds having cleavable cyclic endgroups.

According to another preferred way of implementing the method accordingto the invention, the precursor of the organic film is chosen fromdiazonium salts, especially aryl diazonium salts, sulfonium salts,phosphonium salts, iodonium salts and mixtures thereof, these saltspreferably being functionalized by macromolecular fragments of thepolyethylene and other polyolefin or polyethylene oxide type, and moregenerally any thermoplastic oligomer or polymer.

These salts have in common the fact that their reduction results inradicals that are adsorbed on the conductive or semiconductive surfaceand cause no growth of polymeric chains. This is therefore oneparticular case of electroinitiated grafting, in which the latter isreduced to its simplest expression and allows films of very smallthickness, close to a molecular monolayer, to be produced.

According to the invention, it is also possible to carry out theelectrografting of the organic film by using several precursors chosenfrom the various types of precursors mentioned above.

Be that as it may, the electrografting of the organic film is preferablyobtained by immersing the conductive or semiconductive surface in asolution containing the precursor or precursors of said organic film andby connecting this surface to a potentiostat so as to apply one or moreelectrical potential scans to it, these scans possibly being continuousor discontinuous, sinusoidal or pulsed scans.

Once the organic film has been electrografted, the operation of bondingthe polymer surface to the conductive or semiconductive surface may becarried out, this preferably consisting of a hotmelt operation or a coldbonding operation or else a hotmelt/cold bonding combination.

The hotmelt operation may be carried out by applying one surface to theother and supplying, to the resulting assembly, optionally in a press orclamped, thermal energy, for example by heating or by applyingelectromagnetic radiation, sufficient to melt the two contactingsurfaces and thereby causing them to interpenetrate.

Sufficient thermal energy corresponds, for example, to a temperatureabove that one which, of the glass transition temperatures of thepolymer surface that has to be bonded and of the organic filmelectrografted onto the conductive or semiconductive surface, is thehigher.

As regards the bonding, it is preferred to use, as bonding agent, asubstance, for example a solvent, that is capable of both dissolving orswelling the polymer surface that has to be bonded and the organic filmelectrografted onto the conductive or semiconductive surface.

This bonding may be accomplished in various ways depending on its enduse. Thus, for example, if the purpose of the bonding is to coat apolymer onto a part made of a conductive or semiconductive material andable to withstand being subjected to an immersion operation—which isespecially the case for a stent or a pacemaker package—then this bondingmay be carried out by immersing this part in a solution containing thepolymer to be bonded and the bonding agent and then by drying saidsolution, in which case the formation of the polymer film and itsbonding to the conductive or semiconductive surface take placesimultaneously. When such an immersion operation is not possible, ifonly because the polymer surface is that of an object already formed,then the bonding may be carried out by coating the polymer surface andthe conductive or semiconductive surface with a bonding agent, thenapplying one surface to the other and drying the resulting assembly,optionally under reduced pressure and/or in a press or clamped.

The polymer constituting the polymer surface that has to be bonded maybe purely organic or hybrid (i.e. organic/inorganic) and may be athermoset or a thermoplastic, as long as, in the latter case, it can bedissolved or swollen by a substance that is also a solvent or a swellingagent for the material forming the electrografted organic film.

Moreover, when the electrografted organic film is itself a polymer film,the polymer constituting the polymer surface that has to be bonded maynot only be the same as the polymer constituting this organic film butalso one that differs therefrom.

Appropriate polymers are especially polyethylenes, polypropylenes,polystyrenes, polyacrylonitriles, polysiloxanes, polyesters, such aspolylactic acid and polyglycolic acid, polyorthoesters,polycaprolactones, polybutyrolactones, polyacrylics, polymethacrylics,polyacrylamides, epoxide resins, ABS resins, polyvinylchloride,polycarbonate, polytetrafluoro-ethylene, perfluorinated polyethers,phenoplast resins, polyurethanes, epoxy resins, copolymers thereof andblends thereof.

Within the context of the present invention, it is preferred to usehotmelt polymers—or thermoplastic polymers—even though thermosettingpolymers may also constitute useful candidates.

As regards the material that can form the conductive or semiconductivesurface, this may be any known material that has the properties of anelectrical conductor or semiconductor, such as metals (noble orotherwise) and metal alloys, silicon, germanium or even galliumarsenide.

The method according to the invention has many advantages.

Specifically, it affords, in the first place, the possibility of bondinga polymer material to a conductive or semiconductive material, i.e. amaterial having a very high melting point for which there is generallyno temperature range permitting thermal bonding to a polymer, polymersfor the most part decomposing at the melting point of conductive orsemiconductive materials.

It also affords the possibility of bonding a first conductive orsemiconductive material, coated with an organic film, whetherelectrografted or otherwise, to a second conductive or semiconductivematerial that is not coated with an organic film. In this case, it issufficient to pretreat the surface of the second material so as to coatit with an electrografted organic film and then to carry out anoperation to bond the two conductive or semiconductive materialstogether, as described above. As a variant, the bonding operation may becarried out after having not only electrografted an organic film ontothe surface of the second conductive or semiconductive material, butalso having inserted a polymer film between the organic films formingthe surfaces of the two conductive or semiconductive materials. Thus,when the organic film forming the surface of the first conductive orsemiconductive material is itself an electrografted film, two bondedjoints according to the invention are produced.

In all cases, it turns out that the best results are obtained when theorganic film forming the surface of the first conductive orsemiconductive material is an electrografted film, the electrograftingmaking it possible to have stronger interfaces than with the othermethods currently available for forming an organic film on a surface.

On the basis of the foregoing, the method according to the inventionalso allows two conductive or semiconductive materials, neither of whichis coated with a polymer film, to be bonded together.

In particular, it allows a bonded joint to be produced between twomaterials having very high melting points—this is particularly the casefor metals, silicon and germanium—without ever having to carry out astep involving such a high melting point. This advantage may prove to bevery useful when two conductive or semiconductive regions belonging toseparate and complex objects, certain regions of which areheat-sensitive, have to be bonded together. It also contributes tosubstantial energy consumption savings being made, since it replaces abonding operation that has to be carried out at very high temperatureswith an electrografting operation, which is carried out at roomtemperature, and an operation of thermally bonding the two organicmaterials, which requires much lower temperatures than the thermalbonding of inorganic materials, or even a cold bonding operation.

As mentioned above, the organic film electrografted onto the conductiveor semiconductive surface may or may not be formed from the same polymeras that forming the polymer surface to which said conductive orsemiconductive surface is intended to be bonded.

In the case when the polymers are the same, the method according to theinvention allows thick to very thick films to be obtained very easily ona conductive or semiconductive material, particularly macroscopic films(i.e. those with a thickness of greater than 500 μm) that are highlyadherent, whereas electrografting alone results in the formation offilms having a thickness not exceeding the order of one micron. Thefilms thus obtained may, for example, be plastic films produced byextrusion or any other means, which are then bonded to the conductive orsemiconductive material onto which a film of the same nature has beenelectrografted beforehand.

If the polymers are different, the method according to the inventionmakes it possible to obtain, on a conductive or semiconductive surface,a polymer film which normally would be unable to be electrografted ontothis surface or could only be so with serious difficulties. This isbecause electrografting is a complex process that does not permit allthe polymers to be grafted with the same effectiveness onto allconductive or semiconductive surfaces. In addition, certain polymers,such as those obtained by polycondensation of monomers, lend themselvespoorly to electrografting. The method according to the invention offersa solution to this problem since all that is required is toelectrograft, onto the conductive or semiconductive surface, a polymerfilm which is both compatible with the polymer with which it wasinitially sought to coat this surface and is easy to electrograft ontosaid surface.

In all cases, the method according to the invention makes it possible tocreate very strong adhesive links between the bonded surfaces.

In addition to all the abovementioned advantages, the method accordingto the invention also has other advantages, in particular the following:

-   -   it does not require any prior modification, and especially no        prior functionalization, of the polymer surface that has to be        bonded, thereby, in the case of a biocompatible polymer,        eliminating the risk of it losing its biocompatibility        properties,    -   it does not use any chemical compound other than the precursor        of the electrografted organic film and, when appropriate, the        bonding agent, which considerably limits the risks of toxicity        in the case of biomedical applications, and    -   it is simple to implement and requires neither complex and        expensive operating protocol nor complex and expensive        equipment.

Consequently, the method can be used in very many applications, amongwhich mention may be made of the manufacture and renovation ofcomposites intended for the aerospace, aeronautical, automotive,biomedical, electronic and microsystems industries, the manufacture ofimplantable surgical and medical instruments, the assembly of sensitivecomponents of microsystems and the packaging of Microsystems.

The subject of the invention is also a structure comprising a conductiveor semiconductive surface bonded to a polymer surface via an organicfilm with a thickness of less than 1 μm.

Yet another subject of the invention is a structure comprising aconductive or semiconductive surface bonded to a polymer surface via anorganic film, in which said organic film is bonded to said conductive orsemiconductive surface via covalent bonds.

Such structures are, for example, implantable surgical or medicaldevices such as stents, aneurysm guides, catheter guides, pacemakers,hip prostheses, cochlear implant electrodes, dental implants or evenelectrophysiology electrodes, or else microsystems such as microsensors.

Apart from the above provisions, the invention also includes otherprovisions that will become apparent from the rest of the descriptionthat follows, which relates to illustrative examples of bonded jointsformed by the method according to the invention and examples of theirperformance, this description being given purely by way of illustrationbut implying no limitation, with reference to the appended drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 corresponds to two photographs, respectively A and B, taken in anoptical microscope at two different magnifications (50× and 100×),showing the contact region between a gold wire and a gold slide that arebonded together by hotmelt bonding according to the invention.

FIG. 2 corresponds to two photographs, respectively A and B, showing thebonding region between a polystyrene film and a stainless steel stripthat are bonded together by hotmelt bonding according to the invention.

FIG. 3 is a photograph illustrating the capability of a hotmelt bondedjoint produced according to the invention between two stainless steelstrips to withstand an attempt to separate these two strips.

FIG. 4 shows the pentoxifylline release profiles obtained for twopolylactic acid (PLA) films filled with 20% and 40% (w/w) pentoxifyllinerespectively and bonded, by bonding according to the invention, tostainless steel strips (curves 2 and 3), and the profile obtained for aPLA film filled with 20% (w/w) pentoxifylline and having been depositedon a stainless steel strip (curve 1).

FIG. 5 shows the spectrum, obtained using infrared reflection absorptionspectroscopy (IRRAS), of a polyorthoester (POE) film deposited on astainless steel strip and having a thickness of 500 nm.

FIG. 6 shows the erosion profiles of a POE film bonded by bondingaccording to the invention to a stainless steel strip (curve 2) and of aPOE film deposited on a stainless steel strip (curve 1).

EXAMPLE 1 Bonding by Hotmelt Bonding of a Gold Wire to a Gold Slide

Butylmethacrylate (BUMA) was dissolved, in a three-electrodeelectrochemical cell, in a solution comprising 5×10⁻² mol/l oftetraethylammonium perchlorate in dimethylformamide (DMF), in an amountof 5 mol of butylmethacrylate per liter of solution.

Next, a glass slide coated with an evaporated gold layer was immersed inthis solution. This slide was connected to the working terminal of apotentiostat and acted as working electrode. The other two electrodes ofthe device were a large platinum electrode, serving as counterelectrode,and a silver wire used as reference electrode.

Next, ten potential scans were applied to the gold slide undervoltammetric conditions, between −0.1 and −2.6 V/(Ag⁺/Ag), at a rate of100 mV/s. The slide was rinsed with DMF and then with acetone, andfinally dried in a stream of argon. A film of polybutyl-methacrylate(poly-BuMA) approximately 50 nm in thickness was thus obtained.

The same treatment was applied to a gold wire 25 μm in diameter and 3 cmin length. IRRAS spectroscopy was used to check that the wire wasactually coated with poly-BuMA, but its precise thickness was difficultto determine.

The wire was then deposited on the slide and held in place by means of aMohr clamp. The assembly was placed overnight in an oven heated to 200°C., this representing a temperature well below the melting point of gold(1064.43° C.).

After cooling and removal of the Mohr clamp, it was found that the goldslide could be picked up by merely taking hold of the wire, proving thatthe bond established between this slide and the wire was strong.

FIG. 1 shows two photographs taken under the optical microscope atregions where the hotmelt bonding between the wire and the slide areapparent, photograph A corresponding to a magnification of 50× andphotograph B corresponding to a magnification of 100×.

This figure shows that the number of anchoring points is, however,generally quite small, which is probably due to the fact that the wireis not straight, and is unable to be in contact with the slide over itsentire length during the hotmelt bonding.

EXAMPLE 2 Hotmelt Bonding of a Polystyrene Film to a Stainless SteelStrip

A 316L stainless steel strip 10 cm in length and 1 cm in width wasdipped into a solution containing 3.125 mol/l of methyl methacrylate(MMA), 10⁻2 mol/l of 4-nitrophenyldiazonium tetrafluoroborate and2.5×10⁻² mol/l of sodium nitrate in the DMF. This strip served asworking electrode in a three-electrode arrangement similar to that usedin Example 1.

This strip was subjected to a series of fifty potential scans undervoltammetric conditions, between −0.1 and −3.0 V/(Ag⁺/Ag), at a rate of100 mV/s. The strip was then rinsed with DMF, then with acetone andfinally dried in a stream of nitrogen. A film of polymethyl-methacrylate(poly-MMA) approximately 300 nm in thickness was obtained.

Next, a polystyrene film 10 cm in length, 1 cm in width and 75 μm inthickness was applied to the entire strip thus treated. The film waspressed onto the strip at one of its ends using a Mohr clamp, so thatthe pressed region measured about 2 cm in length by 1 cm in width. Theassembly was placed in an oven at 200° C. for two days.

After cooling and removal of the Mohr clamp, the polystyrene film wasfound to be bonded to the stainless steel strip. In particular, it wasfound to be possible to lift up the strip by merely taking hold of theassembly via the unbonded end of the polystyrene film, as illustrated byphotographs A and B in FIG. 2.

EXAMPLE 3 Hotmelt Bonding of a Polystyrene Film to a Stainless SteelStrip

Using the same protocol as that described in Example 2, polystyrenefilms 75 μm in thickness were bonded to 316L stainless steel stripspretreated by electrografted films of polymethacrylonitrile (PMAN),polyhydroxyethyl methacrylate (PHEMA) and poly-ε-capro-lactone (PCL)respectively, having thicknesses of between 300 and 500 nm.

Results similar to those reported in Example 2 were observed, namely thefact that the bond between the polystyrene films and the strips wassufficiently strong for them to be able to be lifted up by taking holdof the assemblies via the unbonded end of these films.

EXAMPLE 4 Hotmelt Bonding of Two 316L Stainless Steel Strips

Two 316L stainless steel strips 10 cm in length and 1 cm in width werepretreated, in an identical manner, with an electrografted poly-MMA filmapproximately 300 nm in thickness using a protocol similar to thatdescribed in Example 2.

One strip was superposed on the other and, inserted between them, at oneof their ends, was a polystyrene film 2 cm in length, 1 cm in width and75 μm in thickness. The sandwich thus obtained was clamped, using aG-clamp, over an area measuring about 2 cm in length by 1 cm in width.

The assembly was placed in an oven at 200° C. for two days.

After cooling and removal of the G-clamp, it was observed that the twostrips were bonded together. The assembly was placed on the edge and thetwo unbonded ends of the two strips were separated. A spacer wasinserted into the space thus made so as to maintain a separation of 1 cmbetween the unbonded ends of the strips. FIG. 3 shows that the bondedjoint did not fail when stressed in this way.

EXAMPLE 5 Cold Bonding of a Polylactic Acid Film to a 316L StainlessSteel Strip

This example illustrates the benefit provided by cold bonding, carriedout using the method according to the invention, between a polylacticacid (PLA) film and a 316L stainless steel surface, on the stability ofthe interface resulting from such bonding.

This benefit was demonstrated by cold bonding two PLA films, one filledwith 20% (w/w) pentoxifylline and the other with 40% (w/w)pentoxifylline, to two 316L stainless steel strips pretreated with anelectrografted poly-BuMA film, and by comparing the amount ofpentoxifylline released by these films, when the strips were maintainedfor several days in an aqueous solution, with the amount released by afilm of PLA filled with 20% (w/w) pentoxifylline, said film having beendeposited on a 316L stainless steel strip that had not been pretreatedwith an electrografted poly-BuMA film.

To do this, a poly-BuMA film approximately 300 nm in thickness waselectrografted onto two 316L stainless steel strips 10 cm in length and1 cm in width, using the same operating protocol as that described inExample 2.

Moreover, a 10% (w/w) solution of polylactic acid[poly(2-hydroxypropionic acid)], with a weight-average molecular weightof 250,000 g/mol, in chloroform was prepared, chloroform being a solventfor PLA. Starting from this solution, two PLA solutions containing 20%and 40% (w/w) pentoxifylline respectively were prepared. These solutionswere stirred for two hours and then a PLA film filled with 20 or 40%(w/w) pentoxifylline was deposited on the stainless steel strips coatedwith poly-BuMA electrografted by dipping these strips into saidsolutions. The films thus obtained had a thickness of about 3 μm. The316L stainless steel strip not pretreated with an electrograftedpoly-BuMA film was also immersed in the 20% (w/w) pentoxifyllinesolution.

The strips thus prepared were put in an oven at 40° C. for 4 h. Each ofthem was then introduced into a closed container, containing aqueous PBSbuffer solution at 7.4 pH and placed in an incubator at 37° C. withstirring.

Regular samples were taken from the aqueous solution in which each stripwas immersed, said solution being replenished at the same time. Eachsample was extracted with chloroform and the pentoxyfyllineconcentration present in the solution was determined by UV-visiblespectroscopy in transmission at 278 nm.

The results obtained are illustrated in FIG. 4, in which the cumulativeconcentration (in percent) of pentoxifylline released is plotted on theY-axis and time (in hours) is plotted on the X-axis, curves 1, 2 and 3corresponding, respectively:

-   -   Curve 1, to the PLA film filled with 20% (w/w) pentoxifylline        deposited on the untreated stainless steel strip;    -   Curve 2, to the PLA film filled with 20% (w/w) pentoxifylline        bonded to one of the two stainless steel strips pretreated with        an electografted poly-BuMA film; and    -   Curve 3, to the PLA film filled with 40% (w/w) pentoxifylline        bonded to one of the two stainless steel strips pretreated with        an electrografted poly-BuMA film.

This figure shows that release of pentoxifylline by the PLA filmdeposited on the unpretreated stainless steel strip is rapid, since aplateau is reached after 8 to 10 days of incubation, indicating thisrelease has stopped. Inspection of that strip showed that the PLA filmhad delaminated and no longer adhered to the stainless steel surface.

Curves 2 and 3 show that, after a very rapid “burst” of release due todiffusion of the excess pentoxifylline, the release profiles forpentoxifylline released by the PLA films bonded to the stainless steelstrips pretreated with an electrografted poly-BuMA film become linearand ensure steady delivery of this compound. Inspection of these stripsshowed, moreover, no local deterioration (for example flaking) of thePLA films. This means that the PLA/poly-BuMA interface was strong enoughto withstand the release medium and suggests that release would be onlydue to progressive hydrolysis of the PLA at its surface in contact withthe aqueous solution.

Moreover, this example demonstrates that the method according to theinvention makes it possible for a biocompatible polymer film such as PLAto be firmly attached to a metal surface without having to modify thispolymer beforehand—thereby eliminating any risk of impairing itsbiocompatibility—and, in addition, of including in this film a fragilemolecule, such as pentoxyfylline, which is very sensitive to heat and totemperature.

It follows that the method according to the invention can be very usefulfor coating surgical or medical instruments, and especially implants,with biocompatible materials, and in particular with materials intendedfor the controlled release of biologically active substances.

EXAMPLE 6 Cold Bonding of a Polyorthoester (POE) Film to a 316LStainless Steel Strip

This example also illustrates the benefit provided by cold bonding,carried out by the method according to the invention, between apolyorthoester (POE) film and a 316L stainless steel surface, on thestability of the interface resulting from such bonding.

This benefit was demonstrated by cold bonding a POE film to a 316Lstainless steel strip precoated with an electrografted poly-BuMA filmand by comparing the erosion of this film, after the strip was held forseveral days in 9 g/l aqueous sodium chloride solution at 37° C., withrespect to that of a POE film deposited on a stainless steel strip notpretreated with an electrografted poly-BuMA film.

To do this, an approximately 300 nm thick poly-BuMA film waselectrografted onto a 316L stainless steel strip 10 cm in length and 1cm in width using the same protocol as that described in Example 2.

Moreover, a POE₉₅LA₅ (M_(w): 60 000; M_(n): 38 000; T_(g): −14° C.),which is a solid POE, was prepared as described by M. B. Sintzel et al.(Biomaterials, 19, 791, 1998) [15].

Next, a 5% (w/w) POE solution in tetrahydrofuran (THF) was prepared, THFbeing in fact a solvent for the POE and a swelling agent for thepoly-BuMA. A POE film approximately 500 nm in thickness was deposited,by immersing it in this solution, on the stainless steel strip coatedwith the electrografted poly-BuMA film and on stainless steel strip notpretreated with an electrografted poly-BuMA film.

The strips were dried in an oven at 40° C. for 6 h. They were then leftto incubate according to the same protocol as that described in Example5.

Periodically, the strips were removed, drained and dried in a stream ofargon, and the erosion of the POE films coating these strips wasassessed by monitoring the change in the transmittance of the 1745 cm⁻¹band corresponding to the carbonyl groups of the POE, as measured byIRRAS spectroscopy as a function of time.

FIG. 5, which shows the IRRAS spectrum of a POE film 500 nm in thickness(i.e. the initial thickness of the POE films used in the presentexample) deposited on a stainless steel strip, shows the POE carbonylgroup band located at a wavenumber of 1745 cm⁻¹ and used for monitoringthe erosion of the POE films.

FIG. 6, in which the transmittance (in percent) of the carbonyl groupband is plotted on the Y-axis and time (in days) is plotted on theX-axis, represents the erosion profile of the POE film bonded to thestainless steel strip pretreated with an electrografted poly-BuMA film(Curve 2) and that of the POE film deposited on the unpretreatedstainless steel strip (Curve 1).

This figure shows that the POE film deposited on the untreated stainlesssteel strip is released very rapidly from the surface of this strip.This is in agreement with inspection of the strip, which showed thatthis film had been delaminated.

In contrast, no deterioration of the POE film bonded by cold bonding tothe strip pretreated with an electrografted poly-BuMA film was observed.Progressive disappearance by hydrolysis in contact with the aqueoussolution was simply observed.

These results confirm, should it be necessary, those obtained above inExample 5.

CITED DOCUMENTS

-   [1] EP-A-1 110 946-   [2] WO-A-oo/51732-   [3] FR-A-2 781 232-   [4] E. P. Plueddmann in “Fundamentals of Adhesion”, L. H. Lee    (Editor), page 269, Plenum Press, New York 1990-   [5] U.S. Pat. No. 6,022,597-   [6] U.S. Pat. No. 6,287,687-   [7] U.S. Pat. No. 4,421,569-   [8] U.S. Pat. No. 6,306,975-   [9] WO-A-98/49206-   [10] WO-A-99/16907-   [11] EP-A-0 701 802-   [12] WO-A-98/32474-   [13] U.S. Pat. No. 6,335,571-   [14] FR-A-2 780 200-   [15] M. B. Sintzel et al. Biomaterials, 19, 791, 1998

1. A method of bonding two objects together, one of which has a polymersurface and the other has an electrically conductive or semiconductivesurface, which method is characterized in that it comprises: a) theelectrografting of an organic film onto the conductive or semiconductivesurface; and then b) an operation of bonding the polymer surface to theconductive or semiconductive surface thus grafted.
 2. The method asclaimed in claim 1, characterized in that the electrografting of theorganic film is electroinitiated grafting.
 3. The method as claimed inclaim 2, characterized in that the organic film is a polymer film. 4.The method as claimed in claim 3, characterized in that the polymer filmis obtained from monomers and/or prepolymers that are partly orcompletely functionalized by vinyl groups.
 5. The method as claimed inclaim 4, characterized in that the polymer film is obtained from a vinylmonomer chosen from acrylonitrile, methacrylonitrile, acrylates andmethacrylates, acrylamides and methacrylamides, cyanoacrylates, acrylicacid and methacrylic acid, styrene, vinyl halides, N-vinylpyrrolidone,2-vinylpyridine, 4-vinylpyridine and vinyl-terminated telecheliccompounds.
 6. The method as claimed in claim 3, characterized in thatthe polymer film is obtained from monomers and/or prepolymers that arepartly or completely functionalized by cyclic groups that can be cleavedby nucleophilic or electrophilic attack.
 7. The method as claimed inclaim 2, characterized in that the organic film is obtained fromdiazonium, sulfonium, phosphonium or iodonium salts, or mixturesthereof.
 8. The method as claimed in claim 1, characterized in that thebonding operation consists of hotmelt bonding or cold bonding or acombination of the two.
 9. The method as claimed in claim 8,characterized in that the cold bonding is carried out by means of asubstance capable of dissolving or swelling the polymer surface to bebonded and the organic film electrografted onto the conductive orsemiconductive surface.
 10. The method as claimed in claim 1,characterized in that the polymer constituting the polymer surface ischosen from polyethylenes, polypropylenes, polystyrenes,polyacrylonitriles, polysiloxanes, polyesters, polyorthoesters,polycaprolactones, polybutyrolactones, polyacrylics, polymethacrylics,polyacrylamides, epoxide resins, copolymers thereof and blends thereof.11. The method as claimed in claim 1, characterized in that the polymerconstituting the polymer surface is a hotmelt polymer.
 12. The method asclaimed in claim 1, characterized in that the polymer surface is apolymer film coating a conductive or semiconductive material.
 13. Amethod of manufacturing or renovating composites intended for theaerospace, aeronautical, automotive, biomedical, microelectronics andMicrosystems industries which comprises a step consisting in bonding twoobjects together by the method of claim
 1. 14. A method of manufacturingimplantable surgical and medical instruments which comprises a stepconsisting in bonding two objects together by the method of claim
 1. 15.A method of assemblying sensitive components of Microsystems whichcomprises a step consisting in bonding two objects together by themethod of claim
 1. 16. A structure comprising two objects, one of whichhas an electrically conductive or semiconductive surface and the otherhas a polymer surface, these surfaces being bonded to each other via anorganic film with a thickness of less than 1 μm.
 17. A method ofpackaging of Microsystems, which comprises a step consisting in bondingtwo objects together by the method of claim 1.